1. Field of the Invention
This invention relates in general to earth boring drill bits, and in particular to an improved cutter mounting for a large diameter shaft bit.
2. Description of the Prior Art
In U.S. Pat. No. 4,089,382 issued to Pessier, a cutter mounting system is disclosed for mounting cutters on a large diameter drill bit. The drill bit has a cutter support plate secured to a string of drill pipe. A number of cutter assemblies are mounted to the cutter support plate for engaging the earth face. Shafts can be drilled downward, or reamed upward by pulling the bit through a pilot hole, as shown in the drawing. Each cutter assembly includes a cutter mount secured to the cutter support plate. The cutter mount is generally U-shaped, with a pair of legs spaced radially apart and facing away from the cutter support plate. A vertical slot is formed on each end of each leg. A cutter shell with cutting implememts on its exterior is rotatably mounted on a bearing carrier. The bearing carrier has a lug protruding from each end for engaging a slot in a leg. Each lug and leg have transverse, misaligned, apertures, through which a wedge connector is inserted. The wedge connector expands when tightened, forcing the lug tightly against the bottom of the slot, and securing the bearing carrier to the cutter mount. During drilling, the bearing carrier and cutter shell will be replaced when sufficiently worn. The cutter support plate and the cutter mounts will be reused, with new bearing carriers and cutter shells installed.
During drilling with a cutter mount of this nature, battering of the top of the legs and lugs by the formation have occurred. It is possible for the upper edges of the slot to peen over onto the top of the lug. This would make it difficult for removing the cutter shell and bearing carrier. Also wear on the tops of the leg reduces the life of the cutter mount.